Writing this on Mrs Airspeed's computer:
I was running my Sandisk SSD from a SATA 3 (6GB/sec)
The disk was playing up, so I bought a new Samsung 500GB SSD.
Cloned the Sandisk to the Samsung, all seemed to be OK.
Next step was to power down and reboot with Sandisk disconnected, to force the system to use the Samsung.
Went as far as "Verifying Pool Data........." then sat there.
On, off, check BIOS
Eventually, BIOS started telling me that neither SSD was present.
Hooked a SATA 2 cable to one SSD, and it reappeared.
Tried booting, no deal.
Unplugged all disk cables and reconnected again.
No SSDs in BIOS.
NOT a happy Christmas event.
All the same Happy Christmas one and all!

Hi Charlie,
SSD performing below expectations, and sometimes just dropping out enough to interrupt a programme, but not crashing the system.
I have now discovered that neither of my two SATA3 HDD connections is working, but even when the old or new SSD is plugged into a SATA2 slot, they won't boot anyway. BIOS shows them, and lets me set either as priority boot, but system won't go past Verifying DMI Pool Data....
So either the SATA3s have died, or maybe the Marvell controller chipset, and the system doesn't like booting from a slower or different slot.
I guess I'll have to find someone who can test them. Meanwhile, I may do a fresh W7 install onto the new SSD in the SATA2 slot. That may keep me going till I get local help.